This invention relates to wax and, more particularly, to wax sweating.
Wax is useful for candles and many other products, such as wax paper, crayons, coatings for paper cups, corrugated cardboard containers, board sizing, mold releases, base stock for pour point depressants, etc.
Petroleum wax is primarily comprised of branched and straight-chain paraffins. Paraffin wax is often present in intermediate and heavy oils and separates upon cooling. The removal of paraffin wax is desirable to obtain lubricating oils with satisfactory low pour points. The main product of the dewaxing process is a dewaxed oil with the desired pour point and the by-product is slack wax. The wax produced in the dewaxing step can be deoiled and upgraded to produce saleable wax, such as food grade wax. In the past, wax was mainly considered as a by-product of dewaxing of lubricating oils and lubricants. Today, wax is itself a valuable product.
Slack wax can be deoiled by sweating or solvent dewaxing. Wax sweating is the least common method in use today. During conventional wax sweating, a warm liquid oil-wax mixture, called "slack wax," is chilled to a semi-solid state. Oil is entrapped in the solid wax. The solid wax is subsequently slowly heated in a sweating oven, pan sweater, tank, furnace, or heat exchanger. During sweating, the temperature of the wax in the oven is slowly raised to liquify part of the wax. The liquid wax is referred to as liquid drippings and comprises wax and oil. The initial liquid drippings are relatively rich in oil.
During sweating, the liquid drippings are continuously drained from the oven. The remaining solid wax in the oven is leaner in oil. As sweating continues, the oil content of the bed of solid wax remaining in the oven decreases and the melting temperature of the solid wax increases. Concurrently, the oil content of the liquid drippings decreases and the melting point of the liquid drippings increases. Significantly, the oil and wax contents of the liquid drippings are substantially different than the oil and wax contents of the bed of solid wax remaining in the oven.
A typical sweating oven comprises a vertical, shell and tube heat exchanger. Wax from the lube oil dewaxing units is charged as a liquid to the shell side of the oven, then solidified by running cold water through the tube side. After the wax sets up, the water is heated at a specified rate over a period of many days. As the oven warms, the wax begins to melt. The first liquid fractions to drain from the bed through the rundown line have the lowest melting point and contain the most oil. Conversely, the last liquid to come from the bed has the highest melting point and the least amount of oil. The liquid drippings collect in the bottom of the oven and drain into pans. As each pan becomes full, the wax in the pan is typically tested for its congealing point and oil content. The results of this analysis determine whether the wax in the pan is pumped to Foots oil for catalytic cracker feed, intermediate tankage to be re-sweat in another sweating oven, or to hi-fi feed storage to be processed as finished wax. The sweating process continues until all of the wax in the oven has been melted and collected in the pans.
In conventional practice, sweating continues until all of the wax has been melted from the bed. Unfortunately, it is very difficult to control the quality and composition of the actual wax product. It is determined in part by the size of the pans and the oil content of the charge wax.
It is currently impracticable to remove and analyze samples of the wax remaining in the sweating oven at intervals during the process, not only because of the inaccessibility of the wax in the oven, but also because samples taken from any particular location in the sweater are not necessarily representative of the remainder of the wax. It is, therefore, the usual practice to analyze successive samples of the liquid drippings in the drip pans.
One prior art method employed comprises pouring a sample of the liquid drippings into a melting-point wax bath, allowing it to cool and solidify so that a cake of wax is formed. The cake is then observed under light. If the operator observes a greenish tinge, it indicates to him that the wax in the sweating oven needs further sweating.
In the method of U.S. Pat. No. 2,721,165, sweat streams are sampled by passing ultraviolet light of a wavelength between 240 and 350 millimicrons through the sweat streams until the observed absorptivity of the sampled sweat streams reach a value which corresponds to a predetermined oil content of the wax in the sweater, based upon a correlation of the oil content of the wax and the absorptivity of the sweat streams. Once the selected value is reached, sweating is terminated.
The method of U.S. Pat. No. 2,721,165 and other prior art methods of wax sweating have met with varying degrees of success.
It is, therefore, desirable to provide an improved method of wax sweating.